California's supreme court backed a ban on gay marriage on Tuesday, upholding a voter-approved proposition defining marriage as between a man and a woman, but said the marriages last year of 18,000 same-sex couples were still legal.The argument continued in the streets where gay marriage backers vowed to continue the fight at the ballot box in 2010.The court, which last year unexpectedly opened the door to same-sex unions in the most populous U.S. state, bowed to the majority of California voters who passed the ban known as Proposition 8 last November.

"Shame on California," said Nicky Hilton via Twitter following the March 26 Supreme Court decision to back Proposition 8.

Seriously California, what the f***? I can't believe the ban was upheld...we have to keep fighting!!! Gay marriage will be passed!!! - Meghan McCain via Twitter, May 26.

"I'm sure you heard the prop 8 news. One day when everyone is treated with full equality, we'll look back and realize how wrong this was"said Ellen Degeneres on her Twitter site May 26."If marriage were between two men society could not go on. It's not fear, it's just the way it should be"The argument continued in San Francisco streets, where gay marriage backers vowed to continue the fight at the ballot box in 2010."We will not be moved. Repeal Prop 8!" shouted seated protesters who blocked off an intersection in a show of civil disobedience. Police began arresting them one by one. DeWitt Hoard, 65, a black man who legally married another man in California last year, compared the gay rights fight to one for racial equality. "I've been on this road for a while," he said at the courthouse. "Now we can't marry? Yes we will, it will happen."Social conservatives, too, vowed to fight. "If marriage were between two men society could not go on. It's not fear, it's just the way it should be," said college student Mike Choban, 18.

"Twittering too much is not cool but equality is. California get it together!" said Ashlee Simpson Wentz via Twitter following the Supreme Court decision.

Drew Barrymore addressed the crowd tearfully at the May 26 march in Los Angeles, saying "I will fight with you!"

Kelly Osbourne said on her Twitter blog: "I'm disgusted with California for it's decision to uphold proposition 8! It just makes me sick to think that people cant have equal rights!"

The court said the roughly 18,000 marriages that took place in the state between June and November last year remained valid since the ban was not retroactive. That leaves the state of 37 million people with a tiny group of married same-sex couples that cannot grow.

Gay rights advocates in the case had argued that a simple majority cannot be allowed to strip rights from a minority without undermining civil rights, but Proposition 8 supporters said the populist state constitution gave the majority the right to make fundamental changes through direct vote.

Chief Justice Ronald George characterized the gay rights advocates' position as "it is just too easy to amend the Constitution through the initiative process" and then responded that, like it or not, that was the law. "It is not a proper function of this court to curtail that process," he wrote.

Moreover, Proposition 8 did not end broad protections for same-sex couples to form families, he concluded.

Before the California court's move, a flurry of pro-gay marriage rulings and votes in Iowa and New England this year had appeared to reverse a trend toward banning them in the United States.

Forty-two U.S. states explicitly prohibit same-sex marriage, including 29 with constitutional amendments, according to Human Rights Campaign, a gay rights advocacy group. Gay couples can get married in Massachusetts, Connecticut and Iowa, and several states provide for same-sex unions that grant many of the same rights as marriage.Tuesday's ruling was unlikely to be the last move in what is seen as a pivotal state in U.S. culture wars.

In Los Angeles, gay advocates said they would try to change the state constitution again -- to affirm gay marriage -- in a vote as early as November 2010.

"Because no one has the right to deny another their life, even though they disagree with it, because everyone has the right to live the life they so desire if it doesn't harm another and because discrimination has no place in America, my vote will be for equality and against Proposition 8," said Brad Pitt in 2008.

"YOu and your gay peeps just keep at it..African American president didn't come over night.the battle will end shortly.confident," said Kirstie Alley via Twitter.

Gay rights advocates on the courthouse steps in San Francisco began shouting "Shame on you" on Tuesday as soon as the decision was made public.

Governor Arnold Schwarzenegger asked for peaceful and lawful protests in a statement. He predicted same-sex marriage would eventually prevail in California.

Tony Perkins, president of the conservative Family Research Council, predicted continued success for his group, which has prevailed against gay marriage in every state where the question has been put to a popular vote.

"This is not an issue that is going to go away," he said.

The California court on Tuesday did not back away from its sweeping decision last year, which held that same-sex couples had fundamental constitutional rights and deserved special legal protections as a minority class. Proposition 8 was put to voters as a result of that court decision."Proposition 8 reasonably must be interpreted in a limited fashion as eliminating only the right of same-sex couples to equal access to the designation of marriage, and as not otherwise affecting the constitutional right of those couples to establish an officially recognized family relationship," Chief Justice George wrote.

Because of the limited scope of the measure, he concluded, the constitution could be amended by a simple majority vote rather than requiring a more complex form of changing the constitution called a revision.

George, a Republican who wrote last year's opinion in support of same-sex marriage, distanced himself from voters' views. "Our task in the present proceeding is not to determine whether the provision at issue is wise or sound," he wrote in the opinion released on Tuesday.